Known industrial robots of the articulated arm type generally are provided with a movable robot body arranged on a base. A first arm or upper arm having root and tip portions is rotatably pivoted to the robot body at the root portion. A second arm or forearm having rear and front ends is rotatably pivoted to the top portion of the upper arm at a portion between the rear and front ends. Generally, a wrist assembly arranged at the front end of the forearm includes two or three moving elements which are rotatable about different axes. The moving element forming the output end of the robot is adapted to support, a robot hand, welding gun, painting gun, or other working attachment. The moving elements of the wrist assembly are driven by independent wrist drive units about the corresponding axes. Conventional wrist drive units generally comprise a first sprocket arranged in the upper arm, a drive motor arranged on the upper arm and coupled with the first sprocket to rotate the first sprocket, a second sprocket arranged where the forearm is pivoted to the upper arm, a third sprocket arranged in the tip portion of the forearm and operatively coupled with the corresponding moving element of the wrist assembly, an endless chain for transmitting rotational movement of the first sprocket to the second sprocket, and another endless chain for transmitting rotational movement of the second sprocket to the third sprocket.
In the conventional articulated robots with the wrist drive units as mentioned above, many power-transmission components are used between the drive motors and the moving elements of the wrist assembly. Since a chain generally has low rigidity, known robots in which chains account for a large part of the wrist drive unit have a disadvantage of low rigidity against loads acting on the wrist assembly. Furthermore, conventional robots in which chains account for a large part of the wrist drive unit have a disadvantage of a lower accuracy in positioning of the output end of the wrist assembly due to chain elongation due to wear.